Lens device

ABSTRACT

From an object, a first lens that is a meniscus lens having a convex surface that faces an object, a second lens that faces a concave surface of the first lens, a third lens having a concave surface that faces the second lens, and a fourth lens that is a positive lens having a convex back surface, (1) ν3&lt;ν4, (2) 0.5&lt;Ymax/f&lt;0.8, and (3) Σd&lt;1.5 f are satisfied, where ν3 is an Abbe number of the third lens, ν4 is an Abbe number of the fourth lens, Ymax is a maximum height of an image, f is a composite focal length, Σd is a distance between a first surface of the first lens and a second surface of the fourth lens, the first surface facing the object and the second surface facing an imaging plane, any one surface of the first lens and the fourth lens having a non-spherical surface.

TECHNICAL FIELD

This invention generally relates to a lens device, and moreparticularly, to a lightweight and small-sized lens apparatus that canbe mounted on a portable computer, a mobile telephone, or the like.

BACKGROUND ART

Conventionally, small-sized and lightweight lens apparatuses that aremounted on super compact cameras, mobile telephones, and the like aredisclosed in Japanese Patent Application Publication No. 4-211215 andJapanese Patent Application Publication No. 6-88939. Each of theabove-mentioned lens apparatuses is composed of one or two lenses.However, peripherals of the image are greatly deteriorated in quality,and accordingly, a satisfactory image quality cannot be obtained whenthe above-mentioned lens apparatus is employed in an image sensor fortaking an image having a large number of pixels, more than one millionpixels.

Generally, five or six lenses were required to obtain a sufficientresolution as a lens apparatus in use for a one-quarter-size imagesensor, which is used for taking the image having one to two millionpixels. It was thus difficult to downsize and reduce weight.

In addition, in the case where a field angle is wide, 50 degrees ormore, it has extremely been difficult to correct distortion aberrationor color aberration or coma aberration in the peripherals of the image.

DISCLOSURE OF THE INVENTION

It is a general object of the present invention to provide a lensapparatus that is capable of solving the above-mentioned drawbacks.

According to the lens apparatus of the present invention, the number oflenses is four or less, a distance between a first surface facing anobject and an second surface facing an imaging plane is set to 1.5 f orless.

On the bases of an intersection of an axis (light ray) and a chief rayof most off-axis light rays, the aberration generated by a group oflenses provided in front of the intersection is corrected by anothergroup of lenses provided behind the intersection, and the fourth lensmaintains a position of exit pupil to be longer. It is possible tomaintain an optimal correction of a lateral chromatic aberration andtransverse chromatic aberration by keeping Abbe number of the third andfourth lenses within ranged of given formulas.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will be described indetail with reference to the following drawings, wherein:

FIG. 1 is a structure of a lens apparatus in accordance with a firstembodiment of the present invention; and

FIG. 2 shows a lens aberration in accordance with the first embodimentof the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

A description will now be given, with reference to the accompanyingdrawings, of embodiments of the present invention.

Referring to FIG. 1, the lens apparatus in accordance with the presentinvention includes a first lens 1, a second lens 2, a third lens 3, anda fourth lens 4, which are arranged in an order from an object. Thefirst lens 1 is a meniscus lens made of glass having a convex surfacethat faces the object. The second lens 2 is made of glass, is arrangednext to the first lens 1, and has a convex back surface that faces animaging plane. The third lens 3 is made of polycarbonate-based resin, isarranged next to the second lens 2, and has a concave surface that facesthe object. The fourth lens 4 is made of glass, is arranged next to thethird lens 3, and has the convex surface that faces the imaging plane.The first lens 1 has a first surface that faces the object. The fourthlens 4 has a second surface that faces the imaging plane. Both of thefirst and second surfaces have non-spherical shapes, and are configuredto satisfy following conditions.ν3<ν4  (1)0.5<Ymax/f<0.8  (2)Σd<1.5 f  (3)

In the above-mentioned conditions, ν3 denotes an Abbe number of thethird lens 3, ν4 denotes another Abbe number of the fourth lens 4,Ymax/f denotes a maximum height of the image, f denotes a compositefocal length, and Σd denotes a distance between the first surface in thefirst lens and the second surface in the fourth lens, the first surfacefacing the object and the second surface facing the imaging plane.

Table 1 shows a detailed explanation. TABLE 1 Radius of CurvatureDistance Refractive Abbe Number (ri) (di) Index (ni) (ν1) r1 = 1.034 d1= 0.63 n1 = 1.58913 ν1 = 61.3 r2 = 0.78 d2 = 0.25 Diaphragm r3 = 130.326d3 = 0.64 n2 = 1.58913 ν2 = 61.3 r4 = −1.132 d4 = 0.1 r5 = −0.922 d5 =0.4 n3 = 1.585 ν3 = 30 r6 = −4.255 d6 = 0.03 r7 = −6.055 d7 = 0.95 n4 =1.58913 ν4 = 61.3 r8 = −1.467 d8 = 0.5 r9 = ∞ d9 = 1.0 nf = 1.5168 r10 =∞ Non-spherical Coefficient ε a c r1  1.439127  0.5705e−02 −0.1204e−02r2  2.4248 −0.57017e−01 −0.2326e+01 r3  1.0 −0.79051e−01  0.4611 r4 2.2523 −0.17911 −0.9416 r5 −0.002 −0.2405 −0.52979 r8 −0.0007−0.5558e−02  0.5024e−02

The focal length of the entire lens: f=3.685, FNO=3.5, and the fieldangle: 61.6

In the case where a z-axis is a direction of an optical axis, x-axis isvertical to the optical axis, the light travels in the positivedirection, and ε, a, b, c, and d are non-spherical coefficients, thefollowing formula is described.$Z = {\frac{\frac{x^{2}}{r}}{1 + \sqrt{1 - {ɛ\quad\frac{x^{2}}{r^{2}}}}} + {ax}^{4} + {bx}^{6} + {cx}^{8} + {dx}^{10} + \Lambda}$

The referential number ri in FIG. 1 and Table 1 defines the radius ofcurvature of the i-th surface from the object. In the same manner, thereferential number di defines a distance on the axis between the i-thsurface and the i+1-th surface from the object. The referential numbersn1 through n4 respectively define the refractive index of a d-line inthe first lens 1, the second lens 2, the third lens 3, and the fourthlens 4. The referential numbers ν1 through ν4 define the Abbe numbers.

In addition, next to the fourth lens 4, an IR cut filter 5 is arrangedon the side of an imaging plane 6. A CCD, which is an example ofshooting element, is installed next to the IR cut filter 5 on the sideof the imaging plane 6. Only the imaging plane 6 of the CCD is shown. Alight ray control unit 7 is provided between the first lens 1 and thesecond lens 2. The light ray control unit serves as a lens diaphragm.

A light path in this lens structure in accordance with the presentinvention is also shown in FIG. 1. A chief ray of the light rays havingthe maximum height of the image passes through the vicinity of the lightray control unit 7, which is provided behind the first lens 1. A frontgroup of the diaphragm (the first lens 1 in accordance with the presentinvention) and a back group (the second through fourth lenses inaccordance with the present invention) cancel the aberration each other.

In the lens structure in accordance with the present invention, thesmall-sized, lightweight, and low-cost shooting lens can thus beobtained. An exit pupil of the shooting lens is sufficiently longer thanthe composite focal length, and this compact shooting lens has the widefield angle of 50 degrees or more. In addition, approximately 50 percentof luminance ratio is obtainable in the maximum height of the image, andthe resolution around the peripherals of the image (MTF) is 150 linesper millimeter. The lens apparatus having such a high resolution of 50percent or more is thus obtainable.

FIG. 2 shows aberrations in accordance with the first embodiment of thepresent invention. As shown in FIG. 2, it is possible to obtain the lensapparatus having little spherical aberration, astigmatism, anddistortion aberration. The lens apparatus has little color aberration,which is not shown.

In the lens structure in accordance with the present invention, negativeeffects of the concave lens of the third lens 3 play an important rolein correcting the aberration. The third lens 3 has a concave surfacethat faces to the object. The second lens 2 relays the rays of lightfrom the first lens 1 to the third lens 3. The aberration including thefirst lens 1 and the second lens 2 is absorbed on the concave surface ofthe third lens 3.

With respect to the correction of the color aberration in the lensstructure in accordance with the present invention, the third lens 3 andthe fourth lens 4 cancel each other. The color aberration can becorrected sufficiently by satisfying ν3<ν4.

Tables 2, 3, and 4 show the detailed elements in accordance with second,third, and fourth embodiments, respectively. The lens structure inaccordance with the second, the third, and the fourth embodiments, whichare not shown, are same as that in the first embodiment of the presentinvention. It is possible to obtain the lens apparatus that is capableof correcting the aberrations sufficiently and has the resolution of 150lines per millimeter. The lens apparatus having a high resolution isobtainable. TABLE 2 Radius of Curvature Distance Refractive Abbe Number(ri) (di) Index (ni) (v1) r1 = 1.162 d1 = 0.63 n1 = 1.6935 v1 = 53.3 r2= 0.949 d2 = 0.29 Diaphragm r3 = −21.21 d3 = 0.5 n2 = 1.53039 v2 = 55.8r4 = −1.4 d4 = 0.08 r5 = −0.93 d5 = 0.3 n3 = 1.585 v3 = 30 r6 = 8.541 d6= 0.03 r7 = 5.083 d7 = 0.95 n4 = 1.6935 v4 = 53.3 r8 = −1.52 d8 = 0.5 r9= ∞ d9 = 1.0 nf = 1.5168 r10 = ∞ Non-spherical Coefficient ε a c r1 1.704343  0.10247e−01  0.72515e−03 r2  3.13227 −0.15884e−01 −0.95365 r3 1.0 −0.39518  0.152767 r4  4.20229 −0.249413 −0.170572e+01 r5  0.026948−0.393033 −0.1555e+01 r6  1.0 −0.2497e−01 −0.15731e−01 r7  1.0 0.24118e−01  0.7077e−02 r8 −0.009549  0.731e−02  0.2944e−01

The focal length of the entire lens: f=3.682, FNO=3.5, and the fieldangle: 66.7

In accordance with the second embodiment of the present invention, thesecond lens is made of cycloolefin-based resin. The third lens is madeof the polycarbonate-based resin. The first lens 1 and the fourth lens 4are made of glass. TABLE 3 Radius of Curvature Distance Refractive AbbeNumber (ri) (di) Index (ni) (v1) r1 = 1.054 d1 = 0.65 n1 = 1.58913 v1 =61.3 r2 = 0.927 d2 = 0.21 Diaphragm r3 = 16.874 d3 = 0.7 n2 = 1.53039 v2= 55.8 r4 = −1.124 d4 = 0.1 r5 = −0.896 d5 = 0.5 n3 = 1.585 v3 = 30 r6 =−13.972 d6 = 0.04 r7 = −5.207 d7 = 1.02 n4 = 1.58913 v4 = 61.3 r8 =−1.273 d8 = 0.5 r9 = ∞ d9 = 1.0 nf = 1.5168 r10 = ∞ Non-sphericalCoefficient ε a c r1 1.086439  0.27211e−01  0.445e−01 r2 2.52395−0.49324e−01 −0.205717e+01 r4 2.13567  0.15612 −0.142107 r6 1.0−0.72885e−01  0.7911e−02 r8 0.30816 −0.409e−03  0.4196e−02

The focal length of the entire lens: f=3.678, FNO=3.5, and the fieldangle: 61.3

In accordance with the third embodiment of the present invention, thesecond lens is made of cycloolefin-based resin. The third lens is madeof the polycarbonate-based resin. The first lens 1 and the fourth lens 4are made of glass. TABLE 4 Radius of Curvature Distance Refractive AbbeNumber (ri) (di) Index (ni) (v1) r1 = 1.045 d1 = 0.63 n1 = 1.58913 v1 =61.3 r2 = 0.887 d2 = 0.25 Diaphragm r3 = −15.547 d3 = 0.64 n2 = 1.58913v2 = 61.3 r4 = −1.422 d4 = 0.1 r5 = −1.042 d5 = 0.4 n3 = 1.585 v3 = 30r6 = −11.164 d6 = 0.03 r7 = −9.921 d7 = 0.95 n4 = 1.58913 v4 = 61.3 r8 =−1.329 d8 = 0.5 r9 = ∞ d9 = 1.0 nf = 1.5168 r10 = ∞ Non-sphericalCoefficient ε a c r1  1.400562  0.18058e−01  0.27879e−01 r2  2.94814−0.7715e−02 −0.146311e+01 r3  1.0  0.102458  0.21433e+01 r4  2.66328 0.113946 −0.121192e+01 r5 −0.037086 −0.197711 −0.1162e+01 r6  1.0−0.576e-01  0.38232e−01 r7  1.0  0.79477e−01  0.7293e−03 r8  0.018252−0.78839e−03  0.18164e−01

The focal length of the entire lens: f=3.685, FNO=3.5, and the fieldangle: 61.6

In accordance with the fourth embodiment of the present invention, thethird lens is made of the polycarbonate-based resin. The first lens 1,the second lens 2, and the fourth lens 4 are made of glass.

In accordance with the present embodiment of the present invention,neither the first surface of the first lens 1 that faces the object northe second surface of the fourth lens 4 that faces the imaging plane hasa spherical surface. However, there is no limitation to theabove-mentioned non-spherical surface. Any one of the first lens 1 andthe fourth lens 4 may have the non-spherical surface.

In accordance with the present invention, it is possible to obtain thelens apparatus made of four lenses that is small-sized, lightweight, andlow-cost. The field angle is at least 50 degrees, the luminance ratio isapproximately 50 percent, and the peripherals of the image also havehigh resolutions.

The present invention is not limited to the above-mentioned embodiments,and other embodiments, variations and modifications may be made withoutdeparting from the scope of the present invention.

1. A lens apparatus comprising: a first lens that is a meniscus lenshaving a convex surface that faces an object; a second lens that faces aconcave surface of the first lens; a third lens having a concave surfacethat faces the second lens; and a fourth lens that is a positive lenshaving a convex back surface, wherein following conditions aresatisfied,ν3<ν4  (1)0.5<Ymax/f<0.8  (2)Σd<1.5 f  (3) where ν3 is an Abbe number of the third lens, ν4 is anAbbe number of the fourth lens, Ymax is a maximum height of an image, fis a composite focal length, Σd is a distance between a first surface ofthe first lens and a second surface of the fourth lens, the firstsurface facing the object and the second surface facing an imagingplane, any one surface of the first lens and the fourth lens having anon-spherical surface.
 2. The lens apparatus as claimed in claim 1,wherein the second lens has a convex back surface that faces the imagingplane.
 3. The lens apparatus as claimed in claim 1 further comprising alight ray control unit provided between the first lens and the secondlens.
 4. The lens apparatus as claimed in claim 1 further comprising anoptical filter provided between the fourth lens and the imaging plane.